Various chemicals and fuels are manufactured using laboratory synthesis or are extracted from natural sources. Examples include pharmaceutical precursors and organic compounds, such as ethanol and other alcohols, fatty acids, sugars, hydrocarbons, acid esters, citric acids, essential oils, ethyl chloride, formaldehyde, glycerin, lactic acid, monosodium glutamate (MSG), peroxides, saccharin, stearic acid, and vinyl acetate. The economic value of such products manufactured in the U.S. is measured in the billions of dollars annually. Naturally occurring chemicals and fuels are produced in natural systems that rely on photosynthesis. Processing biomass through fermentation to produce chemicals or fuels, the most common example being ethanol, is also well known.
Certain microorganisms have been shown to interact electrochemically with electrodes without requiring molecules that shuttle electrons between the electrodes and the microorganisms (see Lovley, D. R. 2006. Bug juice: harvesting electricity with microorganisms. Nature Rev. Microbiol. 4:497-508 and Lovley, D. R. 2008. The microbe electric: conversion of organic matter to electricity. Curr. Opinion Biotechnol. 19:564-571). In some instances microorganisms, such as Geobacter species, have the ability to oxidize organic compounds to carbon dioxide with electron transfer to electrodes. (see Bond, D. R., D. E. Holmes, L. M. Tender, and D. R. Lovley. 2002. Electrode-reducing microorganisms that harvest energy from marine sediments. Science 295:483-485. Bond, D. R., and D. R. Lovley. 2003. Electricity production by Geobacter sulfurreducens attached to electrodes. Appl. Environ. Microbiol. 69:1548-1555. Chaudhuri, S. K., and D. R. Lovley. 2003. Electricity generation by direct oxidation of glucose in mediatorless microbial fuel cells. Nat. Biotechnol. 21:1229-1232). Such microorganisms can produce electrical current in microbial fuel cells.
FIG. 1 is a schematic diagram that illustrates the operation of a microbial fuel cell of the prior art. Microbial Fuel Cells are described in U.S. Patent Application Publication 2008/0286624 A1, published Nov. 20, 2008, the disclosure of which is incorporated herein by reference in its entirety. The article entitled “Microbial fuel cells: novel microbial physiologies and engineering approaches,” by D R Lovley, in Current Opinion in Biotechnology Volume 17, Issue 3, June 2006, Pages 327-332, describes Benthic Unattended Generators or BUGs, which are microbial fuel cells (MFCs) that can harvest electricity from the organic matter in aquatic sediments by oxidizing the organic matter and having the electricity so produced flow from the anode of the fuel cells through a load external to the fuel cell cathode. That article explains that BUGs consist of an anode buried in anoxic marine sediments connected to a cathode suspended in the overlying aerobic water that provides the free (or molecular) oxygen needed to operate the fuel cell.
Conventional microbial fuel cells (MFCs) operate in the following manner: a biofilm of electricigenic bacteria (e.g. Geobacter, Shewanella, etc) forms (or is provided) on the anode. The bacteria metabolize organic compounds such as acetate, with the generation of electrons and protons. The bacteria directly transfer electrons to the anode. The electrons flow through an external circuit to the cathode, thereby generating current. The protons flow though the medium, pass through a selectively permeable membrane to the cathodic chamber, where oxygen is reduced at the cathode, and combines with protons to form water.
In some cases an electrode has been used as an electron donor. FIG. 2 is an illustrative schematic diagram that shows the configuration of a prior art cell that uses an electrode to supply electrons for reduction of chemicals. A primary study of this was in Geobacter sulfurreducens, where an electrode served as electron donor, the electron acceptor was fumarate, and the reduced compound was succinate (Gregory, Kelvin B.; Bond, Daniel R.; Lovley, Derek R., Graphite electrodes as electron donors for anaerobic respiration, Environmental Microbiology. 6(6):596-604, June 2004). Fumarate, also called fumaric acid, or trans-butenedioic acid, is a dicarboxylic acid with the formula HO2CCH═CHCO2H. Succinate, also called succinic acid, or butanedioic acid, is a dicarboxylic acid with the formula HO2CCH2—CH2CO2H, which is produced by hydrogenating the double carbon-carbon bond in fumaric acid.
Other studies, primarily with Geobacter, followed using the electrode as donor for nitrate reduction to nitrite (Gregory, Kelvin B.; Bond, Daniel R.; Lovley, Derek R., Graphite electrodes as electron donors for anaerobic respiration, Environmental Microbiology. 6(6):596-604, June 2004), uranium (VI) reduction to U(IV) (Gregory, K. B., and D. R. Lovley 2005. Remediation and recovery of uranium from contaminated subsurface environments with electrodes. Environ Sci Technol 39(22):8943-8947); and dechlorination of chlorinated organic solvents in groundwater (Sarah M. Strycharz, Trevor L. Woodard, Jessica P. Johnson, Kelly P. Nevin, Robert A. Sanford, Frank E. Loffler, and Derek R. Lovley, Graphite Electrode as a Sole Electron Donor for Reductive Dechlorination of Tetrachlorethene by Geobacter lovleyi, Applied and Environmental Microbiology, October 2008, p. 5943-5947, Vol. 74, No. 19).
In a paper by Shaoan Cheng, Defeng Xing, Douglas F. Call, and Bruce E. Logan, entitled “Direct Biological Conversion of Electrical Current into Methane by Electromethanogenesis” that was published online on Mar. 26, 2009 (http://pubs.acs.org| doi: 10.1021/es803531g), the authors stated that                 . . . we demonstrate that methane can directly be produced using a biocathode containing methanogens in electrochemical systems (abiotic anode) or microbial electrolysis cells (MECs; biotic anode) by a process called electromethanogenesis. . . .        Renewable biomethane is typically produced by methanogens from a few substrates such as acetate, formate, and biohydrogen gas in anaerobic digesters. Based on thermodynamic calculations, methane could also be produced electrochemically through carbon dioxide reduction at a voltage of 0.169 V under standard conditions, or −0.244 V under more biologically relevant conditions at a pH=7, by the reactionCO2+8H+8e−→CH4+2H2O  Rxn(1)        This suggests that methane could be produced without an organic fuel, at about the same potential needed for hydrogen production with an organic fuel (such as acetate)”.        
In particular, it is apparent from this passage that these scientists consider acetate (or acetic acid) to be a fuel to be consumed, and do not consider acetate (or acetic acid) to be a product that one might make in the reaction system that they describe. In fact, the electrons supplied for methane production at the cathode in this system were derived from the oxidation of acetate at the anode. In addition, this paper does not describe the production of any chemical other than methane (i.e., it does not describe the production of any chemical having a plurality of carbon atoms therein), nor does it describe a reaction that generates hydrocarbons and molecular oxygen as products. Furthermore, the reduction of carbon dioxide to methane, a compound with the same number of carbons as the starting material, is analogous to the previously described microbial reduction of fumarate to succinate with an electrode serving as the electron donor. Therefore, the study by Cheng and co-workers did not foresee the possibility of reducing carbon dioxide to form covalent carbon-carbon bonds to produce multi-carbon products.
For compounds that have a high cost of production, it would be advantageous to have a biologically-based production system that is more energy-efficient and economical than current methods of manufacturing these chemicals.
There is a need for systems and methods that can produce hydrocarbon chemicals and molecular oxygen directly using as reagents water and carbon dioxide in analogy to photosynthesis in plants.